Since right now bats don't do much but contribute to lag for those with weaker computers and annoying some people, I think this would be a great time to update bats seeing the next update is the cave update.
Here's a few suggestions:
1º Make bats drop 0-2 XP when killed (Less than cows/chickens/pigs/sheep)
2º Change rabbit hide's name to small hide, bats now also drop 0-1 small hide.
3º Bats have a very small chance to drop a bat tooth which can be used for life drain potion(takes half a heart every 3s and heals you by that amount, lvl 2 potion makes it deal half a heart of damage every 2s)
4º Bat tooth can also be instead to trade with librarian villagers + X amount o emeralds in exchange for a life stealing enchantment which heals hp equal to 15% of damage dealt per level. (Lvl3 max, aka 45% hp drain)
5º Bats could drop a bat ear, bat ear could be used for an ultrasensing potion, which gives the player ultrasensing effect. This effect makes it so that you can see anything that makes the slighest sound(even through walls) such as mobs or even items falling into water, this potion also allows you to detect nearby ores.
6º Bats can also ocasionally drop guano(feces that work as bonemeal, but grow crops instantly with one click) around half the rate a chicken drops an egg, in addition if a bat drops guano on a crop the crop will instantly grow and consume the guano without the need for a player to apply it.
7º Bats now run away from light, this includes torches, means you don't have to deal with them as much and they will often lead you into the spaces that are not as well lit
8º Bats now attack cave spiders and silverfish as they are small bugs which bats like to eat(maybe not cave spiders but silverfishes atleast) (Bats deal 0.5 or 1 heart of damage)
9º Bats are now neutral, and have 50% chance to aggro on you if you hit them(they all attack as a cluster like wolves)
10º Bats are still mostly passive except when you hit a witch near them which causes all nearby bats to aggro on the witch's attacker(they will deal 0.5/1/1.5 hearts of damage depending on difficulty)
I'm not saying all of these suggestions could/should be implemented but personally I'd like option 1 and 2 to be implemented as well as one of the remaining. The behaviorall changes are also just another addition that could give bats a little more personality(well the 7º, 8º and 10º) as well as make you cautious near them(9º and 10º)